usb: gadget: f_hid: fixed NULL pointer dereference

Disconnecting and reconnecting the USB cable can lead to crashes
and a variety of kernel log spam.

The problem was found and reproduced on the Raspberry Pi [1]
and the original fix was created in Raspberry's own fork [2].

Link: https://github.com/raspberrypi/linux/issues/3870 [1]
Link: https://github.com/raspberrypi/linux/commit/a6e47d5f4efbd2ea6a0b6565cd2f9b7bb217ded5 [2]
Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210723155928.210019-1-mdevaev@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_hid.c

@@ -338,6 +338,11 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
 
 	spin_lock_irqsave(&hidg->write_spinlock, flags);
 
+	if (!hidg->req) {
+		spin_unlock_irqrestore(&hidg->write_spinlock, flags);
+		return -ESHUTDOWN;
+	}
+
 #define WRITE_COND (!hidg->write_pending)
 try_again:
 	/* write queue */
@@ -358,8 +363,14 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
 	count  = min_t(unsigned, count, hidg->report_length);
 
 	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
-	status = copy_from_user(req->buf, buffer, count);
 
+	if (!req) {
+		ERROR(hidg->func.config->cdev, "hidg->req is NULL\n");
+		status = -ESHUTDOWN;
+		goto release_write_pending;
+	}
+
+	status = copy_from_user(req->buf, buffer, count);
 	if (status != 0) {
 		ERROR(hidg->func.config->cdev,
 			"copy_from_user error\n");
@@ -387,14 +398,17 @@ static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
 
 	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
 
+	if (!hidg->in_ep->enabled) {
+		ERROR(hidg->func.config->cdev, "in_ep is disabled\n");
+		status = -ESHUTDOWN;
+		goto release_write_pending;
+	}
+
 	status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
-	if (status < 0) {
-		ERROR(hidg->func.config->cdev,
-			"usb_ep_queue error on int endpoint %zd\n", status);
+	if (status < 0)
 		goto release_write_pending;
-	} else {
+	else
 		status = count;
-	}
 
 	return status;
 release_write_pending: